1. Field of the Invention
This invention relates to methods for fabrication of optical fiber and of optical fiber preforms.
2. Description of the Prior Art
Long distance optical communications has recently made great progress, primarily due to the availability of low-loss optical fiber. As is well known, electromagnetic radiation traveling through an optical waveguide, a fiber, is subject to attenuation due to several mechanisms. Although some of these are essentially irreducible, such as the so-called Rayleigh scattering, others can be eliminated or at least substantially reduced. Among the latter is attenuation due to absorption by impurities present in the guiding region of the fiber, and it is with this attenuation mechanism that this application is concerned.
A particularly important absorbing species is the hydroxyl radical (OH.sup.-), which is formed when hydrogen is present in the fiber material. OH present in silica-based optical fiber causes, inter alia, absorption of electromagnetic radiation of wavelengths in the region of current interest for long-distance optical communication, i.e., the wavelength region of about 0.8 to 1.6 .mu.m. The OH absorption peaks in this wavelength region are due to overtones or combination tones of vibrations at longer wavelengths. For instance, the peaks at about 1.38 and 0.95 .mu.m are caused by overtones of the fundamental OH frequency at about 2.75 .mu.m, whereas the peak at about 1.24 .mu.m is due to a combination tone due to OH and a vibration frequency of SiO.sub.2.
Although recently great progress has been achieved in reducing the OH-content of the guiding region of silica-based optical waveguides, the so-called core, by "drying" with, e.g., chlorine, other approaches for reducing the OH-caused loss are clearly of interest. In pinciple, one such other approach comprises shifting the relevant vibration modes of OH to longer wavelengths, thereby reducing the absorption due to OH in the wavelength region of interest. Such a shift to longer wavelengths would occur if a heavier atom could be substituted for hydrogen in OH, since typically such substitution causes a decrease in the vibrational frequencies proportional to approximately the square root of the ratio of the masses. In particular, substituting deuterium for hydrogen would have the desired effect, since the former has approximately double the mass of the latter. Of course, such deuterium/hydrogen (D/H) exchange results in the appearance of OD absorption lines in the relevant wavelength regime. However, these lines are due to higher overtones, and thus weaker by typically 1-2 orders of magnitude.
It is well known that both hydrogen and deuterium diffuse readily in vitreous silica, and that at elevated temperatures, as well as under other conditions (e.g., irradiation with energetic electromagnetic radiation, typically of wavelength .ltorsim.100 Angstroms), deuterium can undergo an exchange reaction with hydrogen. See, for instance, R. W. Lee, The Journal of Chemical Physics, Volume 38(2), pp. 448-455 (1963), and J. E. Shelby et al., Journal of Applied Physics, Volume 50(8), pp. 5533-5535 (1979).
Methods for manufacturing optical fiber preforms and for producing optical fibers therefrom, as well as fiber designs and properties of structures embodying these designs, are well known in the art and will not be discussed herein. For an exhaustive treatment of these and related topics, see, for instance, Optical Fiber Telecommunications, S. E. Miller and A. G. Chynoweth, editors, Academic Press (1979), incorporated herein by reference.